The objective of this research is to determine why cells of the host immune system are often unable to mount an effective response against tumor cells even when those cells bear recognizable distinct tumor antigens, and to determine the effect of radiation on the host-tumor cell interactions. A better understanding of these interactions is essential if we are to effectively utilize the specificity of the immune system in the control of neoplastic growth. Furthermore, the presence of large numbers of host cells within experimental animal tumors may have a substantial effect on the responses to experimental treatment protocols and thus assessment of host-tumor cell interactions is of vital importance to tumor biology studies. We will use multicellular tumor spheroids of EMT6 mammary sarcoma cells, a relatively immunogenic BALB/c mouse tumor and or RIF fibrosarcoma cells, a weakly immunogenic C3H/He mouse tumor to study the ability of host immun cells to recognize tumor antigens, infiltrate tumors and function within a tumor-like microenvironment. Our previous studies have shown that EMT6 in vivo solid tumors, like in vivo implanted spheroids are heavily in-filtrated with host cells. These in situ host cells will be separated by centrifugal elutriation from the tumor cells, quatitated, and the cell types identified morphologically. Subpopulations of lymphocytes will be identified using specific antisera. Functional anti tumor capabilities of the in situ host cell populations will be assessed using a 51Cr release cytotoxic assay and a clone-inhibition assay and compared to the activity of peripheral lymphoid cells. The extent of in vivo damage to the tumor cells (mediated by host cells) will be evaluated by an in vitro clone-forming assay. Preliminary experiments indicate that in vitro culture of both in situ and peripheral host cells results in a marked increase in their tumorcidal activity. We will analyze these cells to determine if their limited in vivo activity is due to lack of stimulation, incomplete activation or active suppression by tumor or other host cells. The effect of a widely used treatment modality, local irradiation, on the type, quantity and function of in situ host cells will be determined to assess whether effective host-tumor cell interactions are enhanced or suppressed following these treatments.